Bubble levels, also known as spirit levels, (herein after “levels”) are instruments designed to indicate whether a surface is horizontal (level) or vertical (plumb). Levels are used by carpenters, stone masons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and various other workers.
Levels indicate whether a surface is level in one direction or two. Common one-directional levels include tubular bubble levels, which feature a slightly curved glass tube that is incompletely filled with a liquid, leaving a bubble in the tube. At slight inclinations the bubble will travel away from the center position, which is usually marked in some manner. Two-dimensional levels include a bull's eye bubble level, which indicates the level of two perpendicular directions. The bull's eye level includes a bull's eye flat-bottomed device with the liquid under a slightly convex glass face which indicates the center clearly. The bull's eye level includes a bubble that travels away from the center position at slight inclinations in two dimensions rather than one, as with the tubular bubble level.